1. Field of the Invention
The present invention relates to a car body of a motorcar and more particularly to an improved car body of a motorcar including a front body section or a rear body section constituting an engine compartment of which skeleton structure is improved so as to reduce vibration of the car body during low speed operation of an engine including idling operation as well as noise (unpleasant buzzing sound) in a passenger compartment generated due to that vibration.
2. Description of the Prior Art
In a motorcar including a car body comprising a front body section, a central body section constituting a passenger room and a rear body section, either of which front and rear body sections are adapted to constitute an engine compartment, vibration generated by rotation of an engine, particularly low frequency vibration during low speed operation of the engine including idling operation causes the front body section or the rear body section to vibrate and such vibration is transmitted further to the central body section whereby vibratory sound (unpleasant buzzing sound) tends to be generated and recognizable in the passenger compartment. Particularly, in a so-called F. F type motorcar (front engine-front drive type motorcar) having its front body section provided with an engine compartment in which an engine is mounted in the transverse direction, that is, in a direction in which its crankshaft extends at right angles to the travelling direction of a motorcar, and also having front wheels driven by the engine, the engine crankshaft extends at right angles to the travelling direction of the motorcar and thereby the direction of changing of engine torque becomes coincident with the direction of bending mode of the car body due to the mentioned specific arrangement of the engine crankshaft in the transverse direction. As a result, bending vibration of the front body section in the vertical direction is transmitted to the central body section whereby vibration and noise (unpleasant buzzing sound) is amplified in the passenger compartment.
Now, description will be made as to a skeleton structure of the front body section of a conventional F.F type motorcar hereunder with reference to FIG. 1.
At each of both the sides of the engine compartment 2 is arranged a side frame 12 serving as a lower longitudinal strut member and an upper skeleton frame 6 serving as an upper longitudinal strut member for a front wheel housing 8. These frames 6, 12 are fixedly connected to a front pillar 4 at its upper end part and at its lower end part, respectively. As is apparent from FIG. 2 which illustratively simulates deformation vibration of a car body including a front body section as constructed with a skeleton structure in the above-described manner, vibration of the engine propagates over the whole car body while a junction point P at which the side frame 12 is connected to the lower end part of the front pillar 4 acts as a center of vibration. It should be noted that the upper skeleton frame 6 extends linearly and has high rigidity, but at upper junction point Q at which the central body section B.sub.c is joined to the front body section B.sub.f, the frame has low rigidity. Thus, the upper junction point Q is caused to deform greatly. As a result, intensive deformation vibration is generated across the passenger compartment 1 and thereby vibration and noise (unpleasant buzzing sound) in the passenger compartment are amplified.